The present invention relates to digital signal processing systems and, in particular, to communications between transceivers having independent oscillators.
Digital data transmission from a transmitter to a receiver requires a variety of digital signal processing techniques to allow the data to be transmitted by the transmitter and successfully recovered by the receiver. In digital wireless telephone systems, a wireless telephone handset unit communicates via digital radio signals with a base unit, which is typically connected via a standard telephone line to an external telephone network. In this manner, a user may employ the wireless handset to engage in a telephone call with another user through the base unit and the telephone network.
Multi-line wireless telephone systems are in use in various situations, such as businesses with many telephone users. Such systems employ a handset that communicates with up to N handsets simultaneously, typically with digital communications schemes, such as a spread-spectrum, time division multiple access (TDMA). In a spread spectrum system, bandwidth resources are traded for performance gains, in accordance with the so-called Shannon theory. The advantages of a spread-spectrum system include low power spectral density, improved narrowband interference rejection, built-in selective addressing capability (with code selection), and inherent channel multiple access capability. Spread-spectrum systems employ a variety of techniques, including direct sequencing (DS), frequency hopping (FH), chirp systems, and hybrid DS/FH systems.
In a TDMA system, a single RF channel is used, and each handset transmits and receives audio data packets as well as non-audio data packets during dedicated time slices or slots within an overall TDMA cycle or epoch. Other communications schemes include frequency division multiple access (FDMA), code division multiplexing/multiple access (CDM/CDMA), and combinations of such schemes, both full and half duplex. Various modulation schemes are employed, such as carrierless amplitude/phase (CAP) and quadrature amplitude modulation (QAM).
Such digital data is often transmitted as modulated signals over a transmission medium, such as the RF channel, in the form of binary bits of data. (Other transmission media often used for digital communications include twisted-pair systems employing asymmetric digital subscriber loop (ADSL) technology or cable modem systems.) The digital data is often modulated and transmitted in complex digital data form, in which the transmitted data comprises symbols from which the original data can be reconstructed by the receiver. Complex digital symbol data typically comprises real (in-phase, or xe2x80x9cIxe2x80x9d) data, and imaginary (quadrature, or xe2x80x9cQxe2x80x9d) data (I, Q pairs). Each symbol of an I,Q pair may be a multi-bit number, and represent a location of a constellation, mapped against a decision region such as a quadrant. Each symbol is mapped or assigned to a prescribed coordinate in a four-quadrant grid-like constellation using a look-up table (e.g., a ROM). A prescribed number of symbols occupy assigned areas in each quadrant, depending on the encoding scheme. Depending on the number of bits/symbol of a given encoding scheme, each quadrant of the constellation contains a number of symbols at prescribed coordinates with respect to quadrature I and Q axes. For example, in the QPSK encoding scheme, each sample has one of four phase positions, one for each quadrant, so that each symbol pair represents two bits of data.
To transmit a given input data value in a complex data system, the input data value to be transmitted is mapped to a symbol pair or pair of coordinates I ,Q of a corresponding constellation point on a complex signal constellation having real and imaginary axes I and Q. These I,Q symbols, which represent the original data value, are then transmitted as part of data packets by a modulated channel. A receiver can recover the I,Q pairs and determine the constellation location therefrom, and perform a reverse-mapping to provide the original input data value or a close approximation thereof.
In a spread spectrum system, each symbol is transmitted by a string of xe2x80x9csub-symbolsxe2x80x9d or xe2x80x9cchipsxe2x80x9d, derived by multiplying the symbol times a pseudo-random number (PN) binary string. Such systems are thus characterized by a chip rate, which is related to the symbol rate by a so-called spread factor (a factor by which the original symbol data rate has been expanded). Spread spectrum systems may also be used, in general, to transmit any digital data, whether in complex format or not.
As noted above, digital data transmission requires a variety of digital signal processing techniques to allow the data to be transmitted by the transmitter and successfully recovered by the receiver. For example, a communications link must first be established, in which the two transceivers lock onto each other, establish synchronization and other system parameters, and the like. The receiver side of a data transmission in a spread-spectrum digital wireless telephone systems employs a variety of functions to recover data from a transmitted RF signal. These functions can include: timing recovery for symbol synchronization, carrier recovery (frequency demodulation), equalization, and gain control. The receiver includes symbol timing recovery (STR), automatic gain control (AGC), carrier tracking loops (CTL), and equalizer loops for each link. Timing recovery is the process by which the receiver clock (timebase) is synchronized to the transmitter clock. This permits the received signal to be sampled at the optimum point in time to reduce the chance of a slicing error associated with decision-directed processing of received symbol values. In some receivers, the received signal is sampled at a multitude of the transmitter symbol rate. For example, some receivers sample the received signal at twice the transmitter symbol rate. In any event, the sampling clock of the receiver must be synchronized to the symbol clock of the transmitter.
Equalization is a process which compensates for the effects of transmission channel disturbances upon the received signal. More specifically, equalization removes intersymbol interference (ISI) caused by transmission channel disturbances. ISI causes the value of a given symbol to be distorted by the values of preceding and following symbols. Carrier recovery is the process by which a received RF signal, after being frequency shifted to a lower intermediate passband, is frequency shifted to baseband to permit recovery of the modulating baseband information. These and related functions, and related modulation schemes and systems, are discussed in greater detail in Edward A. Lee and David G. Messerschmitt, Digital Communication, 2d ed. (Boston: Kluwer Academic Publishers, 1994).
Because each transceiver operates on an independent oscillator, even if the frequencies are the same, the signal transmitted by one receiver is typically received with a xe2x80x9cspinningxe2x80x9d constellation, i.e. a carrier frequency offset, which is detected and accounted for by the CTL. Thus, when one transceiver transmits at a given carrier frequency in accordance with its local oscillator, the receiving transceiver""s CTL downconverts to the lower passband and digitally removes the residual carrier offset. Thereafter the receiving transceiver is able to regenerate the data stream embedded in the transmitted signal. Of course, when the second transceiver transmits data back to the first transceiver, the first transceiver must also apply a CTL to remove the residual carrier offset.
During initial locking to establish a link, this process can delay acquisition of a locked link, at both ends. Once these carrier offsets are known by both transceivers after the link it initially established, subsequent communications are not delayed as much since each receiver side can begin the acquisition using the last carrier offset recovered. However, the initial link process can be delayed due to each transceiver having independent oscillators. In addition, in a multi-line wireless telephone system employing a base unit and a plurality of handsets, each having a transceiver with an independent oscillator, such as a TDMA system, even after the initial links are established, in order to avoid having to re-determine the correct carrier offset for each separate handset when its slot occurs and thus delaying the acquisition thereof, the base must store and keep track of the carrier offsets for each of a plurality of links. This storing and tracking can be complex, expensive, cause delays, or otherwise be undesirable, yet without it acquisition delay increases.
A wireless telephone system having a plurality of wireless handsets and a base unit, the base unit having a base transceiver. Each handset has a handset transceiver for establishing a wireless link over a shared channel with the base unit via the base transceiver, wherein the base transceiver transmits to a given handset transceiver a forward signal at a carrier frequency. Because the base and each handset transceiver operate on independent oscillators, each handset transceiver receives the forward signal having a carrier offset. Each handset transceiver has a receiver having a carrier tracking loop for detecting and removing the carrier offset from the forward signal; a transmitter for transmitting to the base transceiver a return signal; and an oscillator, independent of a base oscillator of the base transceiver on which the carrier frequency is based, for driving the receiver and transmitter of the handset. The handset transmitter comprises a prerotator that prerotates the return signal in accordance with the carrier offset detected by the carrier tracking loop so that the return signal will be received by the base transceiver with substantially no carrier offset